For decades now, we’ve been promised cheap biofuels from algae. But there’s no free lunch. Growing these mini oil factories in vast ponds requires fertiliser and mechanical aeration; and then the oil has to be extracted. It all costs energy and money so the yields need to be high to make it worthwhile.

One promising industrial species is Nannocholoropsis gaditana, which can produce a lipid – the oil and fat energy store – content up to about 60% of the algae’s ash-free dry weight. But Eric Moellering and colleagues at the company Synthetic Genomics Inc in California, wanted to do better.

Starving the algae of nitrogen, paradoxically, boosts oil production. The problem is that the plants also curtail their growth so there’s no net gain. Ever since the 1970s, scientists have been trying to genetically engineer their way out of this quandary. But they’ve had to wait for the right tool: the bacterially derived CRISPR–Cas9 enzyme that has transformed ham-fisted genetic engineering into precision gene-editing.

Moellering’s group identified a gene, called ZnCys, that was deactivated when the algae was starved of nitrogen. When the researchers completely disabled that gene, they saw oil production double, even without starving the algae.

But the algae still grew poorly, so the scientists made use of the finesse of CRISPR–Cas9 to finely edit the DNA code of the ZnCys gene instead of disabling it competely. As they report in Nature Biotechnology this led to doubling oil yield without dampening algae growth.

The final yield was up to five grams per square metre of algae per day.